1. Field of the Invention
The present invention relates to an illuminating optical system for use in an optical source for emitting white light including radiation components of visible range and infrared range.
2. Related Art Statement
In an optical machinery apparatus, for example, an endoscope or a microscope, a xenon lamp, a halogen lamp, and a metal halide lamp, or the like, are widely used as a light source. These light sources generate ultraviolet light and infrared light in addition to visible light. For example, if a viewing subject absorbs infrared light, generally, an infrared cutting filter is, (comprising an infrared absorption filter and an infrared reflection filter) inserted into an optical filter, so as to prevent the viewing subject from being heated by the infrared light.
An example of such an optical apparatusis disclosed in Japanese Utility Model Application Laid-Opened No. 51,411/91, wherein the white light emitted from the xenon lamp is incident on the infrared absorption filter and infrared reflection filter, successively; the radiation components of the infrared range which are included in the white light are reflected and absorbed to remove the infrared light from the white light. The light flux is transmitted successively to the infrared reflection filter and the infrared absorbing filter where it is condensed by the condenser lens, and the condensed light flux is transmitted to the object point through the light guide.
The above infrared absorbing filter absorbs the light of infrared range from the incident light, and transmits the wavelengths of light which are shorter than the infrared range so that instead of absorption, the infrared light is removed. Therefore, if the white light including the infrared light is incident on the infrared absorbing filter, only the infrared light is removed, so that heating of a sample can be prevented if light is illuminated on a sample. Because the infrared absorbing filter itself absorbs the infrared light, it becomes heated. Therefore, the problem arises that the heated amount becomes large and the infrared absorbing filter suffers from clacking and melting in cases of high intensity incidents of light, and/or prolonged illumination of the light.
Meanwhile, the infrared reflecting filter is formed as an interference filter consisting of generally multi-layer film, which is expensive to manufacture. Moreover, the amount of infrared light removed by reflection, as compared with the energy of light being absorbed by the infrared absorbing filter, is small. Even if the energy of light absorbed is small, a problem arises in that the infrared reflection filter becomes deteriorated during the absorption of light, causing a heating of the filter over a long period of time.